The present invention relates to improved miniature electronic parts, i.e. improved lead terminals suitable for semiconductor devices such as integrated circuits, diodes and transistors.
In recent years, it has been required of a semiconductor device to increase its output and to expand its functions, and a variety of research has been made for fulfilling such requirements. Moreover, there has been an extremely severe requirement to provide a device by which improvements in productivity and thus reduction in costs are possible, in addition to sufficient satisfaction of the above-mentioned requirements.
As a structure of a semiconductor device which can keep up the above-mentioned properties to an enough extent and which can be mass produced, a package type obtained by the use of a resin mold is considered to be most promising, and also in preparing such a type of semiconductor device, various processes such as mounting, bonding and plating are utilized.
A lead frame is one of material parts used under severe conditions. For practical use, the lead frame is required to maintain various properties, i.e., electric resistance and surface oxidizability being low; tensile strength being sufficient; ductility being excellent, because of many bending manipulation being carried out; high-temperature property being good, for example, enough mechanical strength mentioned above being maintained even at 250.degree. C. or more; wetting property to a solder being good; change in properties with time and deterioration in weather resistance of a solder being small; and the like.
Now, as a material suitable for the lead frame, there can be taken up copper (Cu) which is high in conductivity, however, is insufficient in high-temperature property and strength. Accordingly, Cu is used together with other elements. In this case, there are problems in the following points. Namely, when Sn and Fe are added as additives to the Cu and the resultant lead frame is soldered, the added elements will diffuse in a used solder, which fact will bring about deterioration in weather resistance of the lead frame. For the purpose of preventing such a deterioration in the weather resistance, Ni plating and Sn plating have been applied in the form of two layers to the surface of the above-described lead frame which is made of an alloy including the Cu.
This necessity of the two plating processes disadvantageously leads to increase in the number of processes and consequently in the cost of the lead frame. Therefore, it is preferable for cost reduction to omit an Ni plating process, if possible.
The present inventors of the present invention have addressed this problem and consequently have been able to accomplish the invention disclosed in Japanese Patent Publication No. 42390/1978. A product obtained according to this disclosed invention in which each small amount of Cr, Zr and Si is added to Cu can satisfy most of the requirements of the characteristics mentioned above. In the case of the disclosed invention, however, since addition of each metallic element is limited to a small amount (0.01 to 0.3% by weight), problems are liable to occur in that the product does not always have sufficient strength, and moreover it is difficult to control such a minor amount. Accordingly, for the solution of these problems, the present inventors have further conducted research, for example, on how to obtain a device capable of sufficiently fulfilling the above-mentioned requirements by adding Cr alone to Cu, and in the case that Cr and Zr are added to Cu, and in the case that other elements are added.
For example, when Cr, Zr, Ni, Fe and Sn are merely added to Cu, the resultant products cannot satisfy all requirements of characteristics such as oxidizability, conductivity, change in properties with time, wetting property to a solder, weather resistance of a solder, hardness, roughness of appearance and high-temperature property.
The reason why such products just described are so poor in characteristics as not to be used practically is that additives such as Ni, Fe, Sn and the like which are added to the Cu for the improvement of strength are undesirably deposited out or concentrated on the surface of each product and these additives bring about problems such as deterioration with time in weather resistance of a solder, reduction in conductivity and degradation in appearance of a used solder.